Tuning Surface Oxidation States of Nickel Oxide for Efficient Inverted Perovskite Solar Cells

Nickel oxide (NiO_x), as an inorganic p-type semiconductor, has been widely adopted as hole-transporting layers in perovskite solar cells. Despite its superior material stability, the poor charge extraction and multi-vacancies greatly restrict the photovoltages and efficiencies. Here, we propose a facile method to tune the surface oxidation states of NiO_x films by a lithium salt treatment for photovoltage enhancement. X-ray photoelectron spectroscopy measurement indicates that the lithium treatment only reduces the Ni³⁺ sites at the top region rather than in the bulk of NiO_x films. This graded distribution allows more efficient charge extraction at the NiO_x/perovskite interface as revealed by photoluminescence studies. Through a combination of capacitance-voltage and drive-level capacitance profiling measurements, we confirm enhanced built-in potentials and decreased interface defect densities in lithium-modified devices. Further modifying the interface with a self-assembly monolayer, the energy offsets at the interface can be largely reduced. Based on these enhanced properties, the modified devices achieve a high-power conversion efficiency of 22.4% (0.07 cm²) with a 120 mV enhancement in photovoltage in comparison with the untreated devices.